Adding attachment to Outlook template, getting "The property does not exist. The field you want to modify is not valid for this type of item."

Using Outlook 2010 on an Exchange, I've created several Outlook templates (.oft) for my team to send to clients.  Some, but not all, are getting an error message "The property does not exist. The field you want to modify is not valid for this type of item." The templates are stored on SharePoint 2007 and have some attachments already in them (generally .pdf).  The users are trying to add additional attachments (such as a .msg, using Insert, Outlook Item).  They are using Cached Exchange Mode (if they don't, the templates don't work for other reasons).  I would appreciate any suggestions.